Tuesday, December 29, 2009
Hey everyone, welcome back from the holiday weekend! Hope you all had a great Christmas if you celebrate that sort of thing, otherwise I hope you had a really great Friday. We had some gorgeous weather here over the weekend, which meant that I was able to get 2 or 3 post-worthy shots. They’re still on the camera, but you’ll see them someday I’m sure. This one’s actually from this last summer though. This is a view of the sunset (duh) over the Olympic mountains and Puget Sound, seen from downtown Seattle. Always beautiful, if the sun and the mountains are actually out.
Today’s picture gives me a chance to talk about one of my biggest pet peeves again: pictures that aren’t quite straight. It’s something that is really easy to take for granted, but is actually really hard to get just right. For a lot of pictures it really doesn’t matter (like yesterday’s picture of a rose) but anytime you have a shot that includes something that you would normally expect to be either straight up and down (like a tree or a building) or straight across (like the horizon), it kills me when it’s obviously tilted. And it doesn’t even need to be tilted by much. Even a degree or two can destroy a picture, especially if it’s something that stretches across the whole frame. This is particularly a problem for me, because for whatever reason my trigger hand seems to be a bit lazy, and it doesn’t want to hold the right side of the camera as high as the left. So it’s always something I have to explicitly ask myself before I hit the shutter: “am I holding it level this time?” In this particular picture I believe I just about nailed it (it’s oh so slightly off, but it’ll do), but there’s actually another variable going on here that can make it even more of a pain in the butt than usual.
In the cases I mentioned before (trees, buildings, the horizon), it’s generally pretty obvious what the “true” orientation needs to be. But what about the case of a lake with a far distant shore? (Or, like in this picture, a sound.) It usually looks right if the far shore goes straight across, but what if the shore is actually curving away from you? Then in reality it should tilt upwards a bit, but in a lot of cases, that will actually make it *look* wrong, even though it actually isn’t. Frustrating, right? I know, it kills me!
I’ll go ahead and finish this post with the standard caveat to the “is it straight?” issue: you can straighten pictures after the fact with software. But I have always claimed, and I still insist, that doing so subtly degrades the picture. Why? Because of math. The image off the camera is a grid of colored pixels. In order to rotate the image, you’re re-mapping those pixels to different locations. Here’s the kicker though: the new locations don’t exactly line up with pixel locations in the image. Meaning, the center of a particular pixel will be moved to a point that’s a little ways between the original pixel location and the location of the pixel next to it. Meaning the new pixel value will need to be calculated as a weighted average of several pixels. Meaning, you’re going to lose at least a little bit of sharpness, as adjacent pixels that once had a certain amount of contrast from each other will now be blended together. Make sense? It’s not a huge issue, and if it’s a question of either straightening an image or not having it at all, definitely go with the straightening. But if you have a choice between a straightened one and one that was naturally straight, I’d go with the natural one every time.
Want a different way of looking at the issue? Okay, here you go. Imagine a sheet of graph paper. Let’s say you want to draw a horizontal line on that paper by filling in squares. Easy enough, right? You just fill in a row of squares. Same with a vertical line. A 45-degree diagonal line kind of works. But any other slope of a line, and you get into averaging. Meaning, you start doing things like filling in two squares next to each other, then you move diagonally for one square, then diagonally again, etc. If you look at it from far away, it looks like a line of the correct slope, but up close it’s clearly kind of a hackjob. So when you straighten an image, you’re doing the same thing. You’re taking lines that were originally at one slope, and tilting them along a grid. So the result will average out to look decent, especially from far away, but you will have lost some detail in the averaging. Long story short, it’s a better idea to just try to take the image straight the first time.
Notes: Canon EOS Rebel T1i, Tamron 28-300 mm VC lens. 1/320s, f/9.0, ISO 100. Focal length: 154mm.